When operated at “low” frequencies, traditional quarter-wavelength antennas become prohibitively large for certain applications. For example, a quarter-wavelength monopole operating at 10 MHz has a physical size of 7.5 m. This may be acceptable for an outdoor antenna (for instance), but would be impractical for a compact hand-held device. Thus, an antenna designer must employ electrically-small antenna (ESA) techniques in order to transmit and receive signals effectively using an antenna considerably smaller than this natural quarter-wavelength scale.
An ESA is one whose size is on the order of the “radiansphere” or smaller. The radiansphere is the hypothetical sphere of radius λ/2π centered on the antenna. It marks the transition between the near field and far field regions or where energy is stored and radiated around an antenna [H. A. Wheeler, “Fundamental Limitations of Small Antennas,” Proc. IRE, 35, December 1947, pp. 1479-1484].
As a designer shrinks an antenna smaller than quarter-wavelength scale, the design requires reactive loading to ensure that the small antenna resonates at the proper frequency. More reactive loading means more stored reactive energy, and a higher quality factor or “Q.” Q also increases as one reduces loss. A higher Q generally implies a more efficient transmit antenna and a more sensitive receive antenna.
However, the higher the Q, the narrower the bandwidth and the less stable the antenna. Particularly high Q antennas exhibit narrow bandwidth and may be thrown off frequency by changes in their surroundings, temperature variations, or other factors. Antenna designers must make a tradeoff between two mutually exclusive goals: high Q and high efficiency, on the one hand, and stability and bandwidth on the other hand. This fundamental “tyranny of resonance” limits the practical implementation of ultrawideband (UWB), high efficiency, and directional electrically small antenna designs.
In short, there exists a significant need for higher efficiency, electrically small antennas, particularly directive and broadband or UWB small antennas.